To acquire the conventional classification system of their culture, children must learn both the categories that are deemed important or useful, and the category terms that their language contains. Both of these inductive tasks pose many challenges to young children who must categorize and label objects in culturally conventional ways, despite the enormous number of possible ways to do so. Nevertheless, two- and three-year-olds are remarkably capable of acquiring the vocabularies of natural languages. How is it that such young children, with considerable limitations on their attention spans, memory, analytic skills, and other information processing abilities, are able to solve these problems? The answer proposed is that children solve the inductive problem involved in acquiring new work meanings in part by means that do not require highly analytic, hypothetical-deductive reasoning. In particular, children effectively focus on the correct meaning of terms because they are biased learners-they consider only some kinds of hypotheses. Children may limit hypotheses according to the taxonomic and whole object assumptions which state that when children hear a novel label refers to the object as a whole, and not to its parts, or substance, or color, etc.c and that it refers to the objects of the same kind of same taxonomic category, and not to objects that are thematically related, and according to the mutual exclusivity assumption which states that children expect each object to have one and only one label. While past findings strongly suggest that children in their third year of life rely on such constraints to guide their language learning, there is as yet no evidence as to whether these constraints are available to children beginning to acquire language. this is the focus of the proposed work. somewhere around 18 months of age children start acquiring words at an extraordinarily fast pace--in some cases several new words a day. this "naming explosion" may mark a qualitatively new way of acquiring language. The proposed work will explore the hypothesis that the appearance of some of the constraints enables this new fast form of learning to take place. Currently the naming explosion is described in terms of the change in rate in which children acquire terms in their productive vocabulary. It is important, however, to determine whether a comparable change occurs in comprehension because many factors other that lexical knowledge can affect what children are capable or motivated to produce. Comprehension provides a better measure of whether a baby has figured out the meaning of a word. Thus, one goal of this work is to determine when babies become capable of such fast word learning, as revealed by the comprehension of words. This in turn will allow for the investigation of whether this new fast form of language learning is made possible by the emergence of constraints on word meanings. Both cross-sectional and longitudinal studies are proposed to chart the developmental appearance of these assumptions and to determine if they appear by the time at which the babies become capable od rapid learning of new words.